This invention pertains generally to digital communications over cabled networks, and more particularly to systems connectable to the lines of a communications network.
Man""s innate desire to communicate has manifest itself in a rich variety of communications networks and formats that span recorded history. Of particular importance throughout civilized history have been communication networks that allow distant communications. The telegraph, a primitive digital communication network, was the earliest form of distant electrical communication over physical cable networks. The telephone network, an analog communication system, to a large extent relegated the telegraph to obsolescence. The main drawback of the telegraph system was not its digital format, but its inferior data rate and specialized (human) transmitters and receivers. The advent of affordable digital (non-human) processors capable of reliably sending or receiving millions of digital symbols each second, coupled with the increasing demand for the interconnection of digital systems, have fueled a phenomenal resurgence in the area of digital communications over the past few decades.
Samuel Morse possessed a unique advantage over the developers of modern digital communications networksxe2x80x94as he developed his communication format, he had no competing standards to contend with. In contrast, several formats have battled for dominance in the digital communications industry. In the medium-speed digital telecommunications arena, two pulse code modulation formats (each having several variations) have emerged as co-winners in this battle; these formats are commonly referred to as the T1 and E1 standards.
The T1 standard predominates throughout North America and some parts of Asia, particularly Japan. This standard defines signaling at a digital data rate of 1.544 million bits per second (bps). A typical T1 signal comprises digitized versions of 24 voice channels, each channel represented by 8,000 8-bit samples per second. A T1 signal transmits 8,000 frames of information per second; each frame contains 193 bits of information made up of one 8-bit sample from each channel, 24 signaling bits, and one synchronization bit. FIG. 1 illustrates the T1 format pictorially. Other variations on this basic T1 theme exist.
In other parts of the world, including Europe and China, E1 is the dominant standard. The digital data rate for the El standard is 2.048 million bps. This standard can support 30 voice channels at 8,000 8-bit samples per second. An additional 2 channels (at time slot 0 and time slot 16) are reserved for signaling and synchronization. Like T1, E1 prescribes 8,000 frames of information per second. Each E1 frame contains 256 bits, versus 193 for T1, with synchronization and signaling handled by 16 bits per frame. FIG. 2 illustrates the E1 format.
Because of the differences in format between E1 and T1, T1 transmitters and receivers have historically required specialized processors to create or interpret T1 signalsxe2x80x94processors that are incompatible with El signals and processors, and vice-versa. Consequently, digital communications equipment suppliers who desire to support both standards have in the past offered separate productsxe2x80x94one tailored for E1 and another tailored for T1. This solution requires that suppliers design, build, and hold in inventory separate integrated circuits and circuit boards for each standard supported.
Integrated circuits now exist with the capability to interpret or construct either a T1 or an E1 bit stream. Despite this advantage, system suppliers have continued to differentiate E1 and T1 hardware. One of the primary reasons for continued differentiation relates to the impedance that a system must present to an incoming or outgoing E1 or T1 line. T1 line impedance is specified at 100 ohms, while E1 supports two impedance standards, 75 ohms and 120 ohms. Thus, even with a common E1/T1 processor, resistors, and sometimes other circuit elements, must be swapped at the board level in order to allow a change in signal format.
It is recognized herein that a complete system that can handle T1, and preferably both E1 transmission standards, without hardware customization either by a supplier or by an end user, is desirable. It is believed that the systems described herein are the first electronically-configurable T1- and E1-compatible digital communication systems in existence. This invention may advantageously allow a supplier to offer a single, versatile solution for a given digital communication application, which the customer may then configure or reconfigure as needed. The invention offers reliability and ease of configuration to the customer, portability of a single unit between systems utilizing different transmission standards, and even the possibility of an on-the-fly reconfigurable or switchable connection. Such a system may be capable of detecting whether an E1 or T1 signal is present by reconfiguring to each standard in turn, attempting to establish lock, and moving on if lock is not established.
In one aspect of the invention, a circuit for communicating digital data over a wire is disclosed. This circuit comprises a processor programmed for use with digital data in at least two different communication formats and an electrical connector for attaching the circuit to a wire for purposes of digital communication. The circuit further comprises an impedance matcher electrically connected between the electrical connector and the processor. This impedance matcher has at least two electronically selectable values of impedance, preferably with impedance values corresponding approximately to each of the impedance values expected by one of the communication formats. The circuit further comprises a controller connected to the processor and the impedance matcher for purposes of electronically selecting one of the communication formats and one of the selectable values of impedance. In a preferred embodiment, impedance values are paired with communication formats, such that only specified combinations of impedance and data format are selectable.
In a further aspect of the invention, electronically-configurable impedance matching circuitry is incorporated directly onto an integrated circuit that contains processor circuitry. This may provide further advantages of reduction in component count, control wiring complexity, and reduced signal reflections. Preferably, such an integrated circuit contains control circuitry for changing both the communication format and impedance in response to a single command.
In yet another aspect of the invention, a circuit which communicates multiple digital data signals simultaneously over separate wires is disclosed. This circuit comprises multiple data processors, each programmed for use with digital data in at least two different communication formats. Each data processor is electrically connected to an impedance matcher, and each impedance matcher has at least two electronically selectable values of impedance. This circuit further comprises control logic for electronic selection of impedance values for each impedance matcher, and a programmable control processor that is connected to both the control logic and the data processors. Preferably, the control processor has the capability to independently select impedance values and communication formats for each pairing of impedance matcher and data processor.